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08_-_bioelectricity_and_membrane_potentials - BMEN E4001x...

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BMEN E4001x: Quantitative Physiology I / Molecular and Cellular Systems Notes 08 - Membrane Potentials and Electrodiffusion Last time, we focused on the role of pore, channel, carrier, and pump proteins in moving compounds with or against a chemical gradient. Many of these compounds are charged ions, so voltages across the membrane will certainly play a part in all of this. For reference, here are the “standard” ion concentrations inside and outside a typical cell. Ion concentration (mM) interstitial space cell (“typical”) Na + 145 15 K + 4.5 120 Ca 2+ 1.2 1 x 10 -7 Mg 2+ .55 1 Cl - 116 20 HCO 3 - 25 15 glucose 5.9 low At some point, you encountered the concept of transmembrane voltage; we’re going to look into this closer, and explore some of the complexities. First of all, how do you measure it, where is it found? Certainly, the central nervous system and neuron activity is a big instance of where these We’ll start here looking at where resting potentials come from, the foundation for more complex behaviors, and explore some of the spatial aspects of this system.
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Start with KCl inside, no KCl outside, 0 volts across the membrane. Note that voltage is read with respect to outside. Membrane permeant to K, but not Cl; channels in action here. Flow of K through the membrane carries positive charge outward, leaving the inside of the cell negative. Now, from the point of view of ions either inside or outside the cell, they see a barrier carrying a slight charge; from the outside, the cell is slightly negative, while from the inside, the barrier is slightly positive. As a result, there is a slight accumulation of K on the outside of the cell, and depletion inside the cell, opposite for Cl. Note: there isn’t a bulk unbalance of charges in solution; charge unbalance can be tolerated at small scales (thanks to thermal kicks), but cannot exist at bulk scales. For example, no solution of Cl - without counterions. How “thick” is this charged layer?
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